Add a serialize method

[emilos]

Context

Sometimes it's useful to serialize an abstract syntax tree node to a value/object.

For example:

{
  type: 'ArrayExpression',
  elements: [
    { type: 'Literal', value: 1 },
    { type: 'Literal', value: 2 },
    { type: 'Literal', value: 3 },
    { type: 'Literal', value: 4 },
    { type: 'Literal', value: 5 }
  ]
}

can be serialized to:

[1, 2, 3, 4, 5]

Acceptance criteria

• install asttv(https://github.com/buxlabs/asttv)
• add a static serialize method
• accept a node, use asstv and return the result
• add specs
• update README